This section introduces aspects that may facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In cellular telecommunications, when a user equipment (UE) moves from a cell to another cell, handover should be performed in order to maintain the communications of the user equipment without interruption. The success rate of the handover is a key aspect in cellular system design.
FIG. 1 illustrates an example procedure for a UE to take handover between two cells in Long Term Evolution (LTE) systems. In FIG. 1, two base stations, e.g. enhanced NodeB (eNB) A and eNB B are shown for illustration. FIG. 1 also shows some cellular coverage areas, also called cells or sectors, served by eNB A and eNB B, respectively. For example, cell 1 and cell 2 are served by eNB A, and cell 3 and cell 4 are served by eNB B. A UE located in the overlapping region of cell 2 and cell 3 is shown to explain the handover procedure from cell 2 to cell 3.
In the exemplary scenario shown in FIG. 1, the UE moves slowly. For example, the UE is carried by a person walking in a pedestrian road.
Normally, according to the interaction with a UE, the handover procedure can mainly comprise three phases. During the first phase, as indicated by the signaling line 1, a measurement report is triggered and sent from the UE to a source eNB (e.g., eNB A in this example) which serves a source cell (e.g., cell 2 in this example). The measurement report may comprise information about signal quality of the source cell and a neighboring cell (e.g., cell 3). Then, the source eNB makes decision based on the measurement report and some other information to hand over the UE to the neighboring cell, i.e., the target cell, which is served by a target eNB (e.g., eNB B in this example). Some necessary information may be exchanged between the source eNB and the target eNB in order to prepare the handover.
During the second phase, as indicated by the signaling line 2, the source eNB sends a handover (HO) command to the UE. The HO command can be a radio resource control (RRC) connection reconfiguration message including mobility control information.
During the third phase, as indicated by the signaling line 3, the UE performs the handover, detaching from the source cell (cell 2) and synchronizing to the target cell (cell 3), as commanded by the source eNB. The UE may access the target cell via random access channel (RACH). When the UE has successfully accessed the target cell, the UE sends a RRC connection reconfiguration complete message to confirm the handover, to the target eNB to indicate that the handover procedure is completed for the UE. Then, the target eNB can now begin sending data to the UE.
With the emergence of various traffic tools, especially the development of high-speed trains, the mobility environment is more complex than ever.
FIG. 2 illustrates an example of a too-late handover of a UE due to high-speed movement. The scenario as shown in FIG. 2 is similar to that shown in FIG. 1, except that the UE in FIG. 2 moves fast. For example, the UE is carried by a person traveling in a high-speed train.
Due to the high moving speed of the train (e.g., more than 250 km/h and up to 350 km/h), there exists a possibility that after the fast-moving UE sends a measurement report in a source cell (e.g., cell 2 in the example shown in FIG. 2), the UE moves out of the source cell fast and into another cell (e.g., cell 3), and hence is unable to receive a handover command sent from the source eNB (e.g., eNB A). This kind of handover failure is regarded as too-late handover, as shown in FIG. 2.
In Reference 1 (WO2013/097063A1), a kind of optimization approach with adjustable handover triggering condition is adopted to solve the above problem. FIG. 3 illustrates an example of an advanced handover for a fast-moving UE according to the disclosure of Reference 1. The scenario as shown in FIG. 3 is similar to that shown in FIG. 2, and the UE in FIG. 3 also moves fast. For example, the UE is carried by a person traveling in a high-speed train.
As shown in FIG. 3, for the fast-moving UE, the handover is triggered in advance. As indicated by the signaling line 1, a measurement report is triggered and sent from the UE to a source eNB (e.g., eNB A in this example) which serves a source cell (e.g., cell 2 in this example), before the UE enters into the overlapping region of the source cell and a target cell (e.g., cell 3). Then, as indicated by the signaling line 2, a handover command sent from the source eNB can be received by the UE before the source eNB cannot reach the UE. Thereafter, as indicated by the signaling line 3, the UE performs the handover as commanded by the source eNB. When the UE has successfully accessed the target cell, the UE sends a RRC connection reconfiguration complete message to confirm the handover, to the target eNB to indicate that the handover procedure is completed for the UE. Then, the target eNB can now begin sending data to the UE.
However, in practice, it is quite possible that some low-speed UEs are also moving in the same area, e.g., when the train is moving in a low speed in some abnormal working mode, or when there is a pedestrian road built parallel to the railway. Then, if the advanced handover triggering condition is applied to the UEs on the pedestrian road, another type of handover failure may occur, which is referred to as a too-early handover. FIG. 4 illustrates an example of a too-early handover of a UE due to low-speed movement. The scenario as shown in FIG. 4 is similar to that shown in FIG. 1, except that the advanced handover triggering condition is applied to the UE with a low speed.
As shown in FIG. 4, for the low-speed UE, the handover is also triggered in advance. As indicated by the signaling line 1, a measurement report is triggered and sent from the UE to a source eNB (e.g., eNB A) which serves a source cell (e.g., cell 2), before the UE enters into the overlapping region of the source cell and a target cell (e.g., cell 3). Then, as indicated by the signaling line 2, a handover command sent from the source eNB is received by the UE. However, due to the low speed of the UE, during the third phase as indicated by the signaling line 3, the UE sends a RRC connection reconfiguration complete message to the target eNB (e.g., eNB B) before the UE enters into the target cell. Thus, the target eNB is unable to receive this RRC message, because the UE has not entered the effective coverage of the target cell, and the handover procedure fails.
To avoid handover failure in such complex scenarios, it is required to distinguish between high-speed UEs and low-speed UEs and then set different handover conditions to them, making sure that they have either earlier or normal handover triggering condition respectively.
Some high-speed/low-speed UE identification methods operated at an eNB are proposed. One of the major used methods is based on physical layer uplink Doppler Frequency-Offset Estimation (FOE). Reference 1 also discloses a method for identifying a high-speed UE in high speed railway, which is based on information of UE past cells and the cell deployment.
However, in practical network operations, it is difficult or even impossible for the operator to manually and correctly configure the UE identification criteria. For example, when Doppler FOE is used to identify UE speed, the criteria are strongly dependent on the distance between the eNB and the railway, the cell size, and even the railway trail line shape.